In the last fifteen years the presence and distribution of large aggregates in the Mediterranean Sea have begun to be studied suggesting that these particles could play a relevant role in the carbon biogeochemistry of the basin. However the massive formation is a sporadic event and it is often difficult to detect. During the autumn of 2000 high densities of mucilage aggregates were observed in the major part of the coastal zone of the Tyrrhenian Sea (western Mediterranean Sea). The chemical composition and origin of these aggregates were evaluated. The carbohydrates, proteins, lipids and the humic and fulvic acid fractions constitute 45– 68% of the organic carbon (OC). Marine origin was suggested by UV spectra and C/N ratios of humic substances extracted from mucilage samples. The polysaccharide component was isolated and purified by extraction of four fractions of different solubility and the saccharide carbon percentage ranged from 13 to 20% of the OC. By comparison the qualitative and quantitative monosaccharide, fatty acid and sterol compositions were very similar to those obtained from Adriatic pelagic mucilages, while on the other hand the results showed remarkable differences in comparison to benthic aggregates. Scanning electron microscopy (SEM) on native samples showed a complex morphology with the presence of fibrillar structures forming a highly branched tri-dimensional network similar to those found in Adriatic pelagic samples.
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